Retroreflective sheets have found use in a wide range of applications, such as traffic signs, guide signs, warning signs, restriction signs, vehicle license plates, advertising signs, and so forth. An example of these retroreflective sheets something is called an enclosed lens type, which comprises at least one surface layer, glass beads with a high refractive index, a focusing layer (also called a focusing resin layer), and a metal reflective layer, which are layered in that order. Another configuration is what is called an encapsulated lens type, which comprises a plurality of transparent spheres provided with a reflective mirror on the lower hemisphere, a support resin sheet for supporting the plurality of transparent spheres, and a transparent cover film that covers the plurality of transparent spheres by being disposed on the surface of the support resin sheet. A joining component that supports the cover film is formed on the support resin sheet. With an encapsulated lens type, because the reflective mirror is formed directly on the surface of the transparent spheres, the reflective brightness at a small observation angle and up to a large incidence angle is markedly superior to that of an enclosed lens type, so this type also is called a high-intensity retroreflective sheet. In the above-mentioned retroreflective sheets, a pressure-sensitive adhesive and a release paper or film further are laminated. Such retroreflective sheets are applied to a substrate, such as an aluminum plate, steel plate, painted steel plate, stainless steel plate, or other such metal substrate, or a plate of fiber reinforced plastic (FRP), hard vinyl chloride, or other such plastic plate, and used as a sign, advertisement board or the like. In the daylight these retroreflective sheets look the same as ordinary signs or a signboard, but at night they accurately retroreflect projected light in the direction of the light source, so they have been useful in greatly enhancing the visibility of the above-mentioned signs, license plates, signboards, and so on.
The retroreflective performance of these retroreflective sheets has been specified by standards in various countries around the world by means of the angle formed by the irradiation axis of projected light and the face center normal line of the retroreflective sheet (the incidence angle) and the angle formed by the irradiation axis of projected light and the observation axis (the observation angle).
However, Japanese Industrial Standard JIS Z 9117 and various standards from around the world only require a maximum of 2° for the observation angle, and a maximum of 50° for the incidence angle.
Therefore, the retroreflective sheets that are currently available on the market are manufactured so as to meet these standards. The minimum requirement for the above signs, license plates, and so forth is to satisfy the reflective performance standards for each country Still, even if these standards are met, there have been problems with commercial products in that with a road sign installed at a right angle to the road, for example, even though the sign may be effective on a straight road, if the incidence angle exceeds 50° at a place where the road bends, there is a pronounced decrease in reflective performance, and the visibility of the sign is drastically reduced. When the application is a vehicle license plate, the observation angle will be large if the driver's seat of a following vehicle is located up high (such as with a truck) and the vehicle in front has a license plate that is located down low, and the incidence angle will be large if the vehicle in front turns to the right or left; in either case this makes it difficult for the license plate of the vehicle in front to be seen from the following vehicle.
Furthermore, it is said that a vehicle traveling at 40 km/h takes from 18 to 22 meters to come to a stop once a hazard is recognized, but when a location 3 meters off the side of the road is passed at 40 km/h, it is said that an incidence angle of approximately 82° is required to confirm the safety of the road side 22 meters ahead by utilizing a retroreflective sheet. In addition, with a conventional retroreflective sheet installed approximately parallel to the road, because of the extremely wide viewing angle, information cannot be conveyed accurately. Consequently, there is a strong market demand for the development of a retroreflective sheet with superior wide angle characteristics.
As a way of dealing with the above, Patent Document 1 below proposes that with the above-mentioned enclosed lens type retroreflective sheet, for example, a focusing resin layer of uniform thickness be formed by using a resin paint to powder coat the upper hemispherical surface of glass microspheres whose lower hemisphere is embedded in a surface resin layer. Patent Document 2 below proposes that a transparent resin film formed ahead of time in a uniform thickness be superposed over the exposed surface of transparent microspheres that have been half-buried in a layer such as a surface resin layer, and this transparent resin film be heated and softened to embed the transparent microspheres securely and form a transparent resin focusing layer. Patent Document 3 below proposes that a retroreflective sheet having less angle dependence and excellent retroreflective performance can be obtained, by embedding in a surface layer a multilayer microsphere structure composed of a transparent focusing layer formed so as to cover the above-mentioned transparent microspheres substantially concentrically in a substantially constant thickness on the surface of the transparent microspheres, and then forming a reflective layer. Patent Document 4 below proposes a retroreflective sheet comprising numerous transparent microspheres embedded in a transparent resin, a focusing resin layer, and a light reflecting layer, in which the light reflecting layer on the top of the back side of the transparent microspheres is located in closer contact to the transparent microspheres than is the focal position of the transparent microspheres, and the light reflecting layer on the lateral back side of the transparent microspheres is at the focal position. Patent Document 5 below discloses an external distant illumination system and method, which in recent years has been gaining popularity in road signs. Furthermore, retroreflective sheets that are already on the market and provide wide-angle visibility include a wave reflector (made by NTW), which is a super-wide angle visual guidance material in which a microprismatic retroreflective sheet is used for the reflection face and the material is formed in a wavy shape, a wide-angle prismatic retroreflective sheet (VIP Grade, made by 3M), and a wide-angle prismatic retroreflective sheet used for side markings (EV-9010, made by 3M).    Patent Document 1: JP S51-128293A    Patent Document 2: JP H8-27402B ((JP S59-198402A)    Patent Document 3: JP H8-101304A    Patent Document 4: JP S58-88202U    Patent Document 5: Japanese Patent No. 2,910,868 (JP H10-506721A (Tokuhyo))
However, the difficulty with the reflective sheet proposed in Patent Document 1 above was how to form the powder of the focusing resin layer in a uniform thickness on the surface of the microspheres. The difficulty with the reflective sheet proposed in Patent Document 2 above was how to bring the film into close contact with the microsphere surface or the surface resin layer in which the microspheres were embedded. With the reflective sheet proposed in Patent Document 3 above, it was extremely difficult to form the required focusing layer film thickness accurately on the surface of the microspheres. There is a certain amount of distribution to the diameter of the microspheres, and the optimal film thickness cannot be obtained for all of the microspheres even if the focusing layer film is formed in a thickness suited to the median diameter. Therefore, this product is far from matching the reflective performance of encapsulated lens type retroreflective sheets in which optimal reflective performance is obtained with all of the microspheres by providing the reflecting layer directly on the individual microspheres. Also, there is no mention of the means or method for achieving the reflective performance at a large observation angle. Therefore, merely forming the focusing layer in a constant thickness at the focus formation position, as was the case with these proposals, was insufficient to ensure wide-angle reflective performance at a larger observation angle and a large incidence angle. If good reflective performance could be obtained at a small observation angle, the above-mentioned encapsulated lens type retroreflective sheets would be satisfactory. Patent Document 5 discloses a sign illumination system and method in which a sign is illuminated from an illumination source installed on the road shoulder away from the sign, but even conventional encapsulated lens type retroreflective sheets that are said to have a wide observation angle do not exhibit good enough reflective performance for use in this illumination system. For instance, in the case of a multilane road, there is a considerable difference between the brightness at which the sign can be seen from a vehicle traveling in the far left lane, and the brightness at which the sign can be seen from a vehicle traveling in the far right lane. Specifically, light emitted from an external flood light can be seen at a relatively small observation angle from a vehicle traveling in the far left lane, so there is a relatively large amount of light that bounces back from the sign and the sign looks brighter, but the observation angle is much larger for a vehicle traveling in the far right lane, which greatly reduces the amount of light that bounces back from the sign and makes the sign look far darker.
Furthermore, a microprismatic retroreflective sheet tends to lose some of its retroreflective performance when the light hits obliquely or from the side, so the above-mentioned wave reflector is designed such that the above-mentioned microprismatic retroreflective sheet is given a wavy shape, which reduces the incidence angle of light irradiated obliquely or from the side with respect to the retroreflective sheet for the light to enter. This does result in an increase in retroreflective brightness, but the retroreflective sheet itself does not have any inherent wide-angle characteristics. Also, since the sheet is formed in a wavy shape, it is extremely difficult to print on a wave reflector by screen printing or another such method, so there is the drawback that the required information has to be incorporated into the retroreflective sheet prior to the wave molding, and the molding performed only after this, and this drives up the cost. In addition, there are large undulating depressions on the reverse side of the wave reflector, and when it is applied to a side wall of a road, dirt and other such foreign matter accumulates in these depressions, which markedly reduces the attractiveness of the sheet.
Also, the above-mentioned wide-angle prismatic retroreflective sheet (VIP) was not designed so that retroreflective performance could be exhibited at a relatively large observation angle and when the light was incident obliquely or from the side. For example, retroreflective performance could not be maintained at an observation angle of 4° and an incidence angle of 40° or more.
With a wide-angle prismatic retroreflective sheet used for side markings (EV-9010, made by 3M), retroreflective performance can be maintained even at a large observation angle and a large incidence angle when the observation point is located in the lateral direction of the sheet (the marking direction), but there is a drastic decrease in retroreflective performance if the incidence angle is over 60° when the observation point is located in the longitudinal direction of the sheet (the vertical direction). When a person is driving a vehicle, the eyes of the driver (that is, the observation point) are located above the headlights, so the retroreflective effect will not be manifested adequately to the driver of the vehicle if one of these wide-angle prismatic retroreflective sheets used for side markings is employed for visual guidance entailing a large incidence angle.
A conventional enclosed lens type of retroreflective sheet will now be described with reference to FIG. 8A. First, a surface layer 10 is produced on a processing substrate. This surface sayer 10 is then coated with a resin solution that forms a glass sphere fixing layer 11. This coating is then dried to disperse the glass spheres 13 in a still tacky state in the glass sphere fixing layer 11. This or another such method is used to bond the glass spheres to the glass sphere fixing layer 11, after which the fixing layer 11 is heated, which submerges the glass spheres 13 and thermally cures the fixing layer 11, thereby sufficiently fixing the glass spheres 13. In the next step, the surface of the glass spheres 13 is coated with a resin solution that forms a focusing layer 12, and this coating is dried. In this case, the glass sphere fixing layer 11 is adjusted to a thickness that is from 50 to 80% of the diameter of the glass spheres 13 when the glass spheres have been submerged. The glass spheres are fixed in a state of just reaching the surface layer. In the FIGS. 15 is a pressure sensitive adhesive layer and 16 is a released material.
There is also an enclosed lens type of retroreflective sheet in which the surface layer 10 and the glass sphere fixing layer 11 are constituted as the same layer, in which case the glass spheres are held submerged in the above-mentioned same layer to up to about 60% of their diameter, from roughly the center of the glass spheres. Here again, the same method as above is employed, in which a focusing layer resin solution is applied and dried.
The focusing layer 12 is thus formed by applying and drying a focusing layer resin solution, and in this case the focusing layer resin solution uniformly coats the entire surface of the sheet and is then dried, so the focusing layer film thickness cannot be adjusted individually for each glass sphere, and the focusing layer is merely formed in a uniform thickness over the entire sheet.
Also, the focusing layer resin solution undergoes volumetric shrinkage when the focusing layer resin solution coating is dried, and this shrinkage stress tries to cause the coating to go around to the back side of the glass spheres and form an ideal concentric circle. If the focusing layer 12 could be formed in a uniform thickness at the focal position of the glass spheres, then as shown in FIG. 8B, incident light b1 from the front would be reflected by the metal reflective layer 14 on the back side of the glass sphere focusing layer, and retroreflected as reflected light b2 substantially parallel to the direction of incidence, while incident light c1 that comes in obliquely also would be retroreflected as reflected light c2 substantially parallel to the direction of incidence. In actual practice, however, the focusing layer solution also comes into contact with the fixing layer between the glass spheres, and the focusing layer solution is attracted to the fixing layer in the course of drying, or flows to a lower position under the force of gravity. As a result, the focusing layer resin is hindered from forming a concentric circle, so the back side of the glass spheres ends up being thinner and the lateral sides thicker, forming the focusing layer shown in FIG. 8C.
Specifically, when the focusing layer thickness on the back side of the glass spheres is made to coincide with the focal position of the glass spheres, incident light d1 from the front is retroreflected as reflected light d2 substantially parallel to the direction of incidence, but incident light e1 that comes in obliquely is diverged from the direction of incidence and retroreflected as reflected light e2. Therefore, optimal reflection is only possible at the small incidence angle indicated by β in FIG. 8C.
Also, as the retroreflective sheet (FIG. 8D) proposed in Patent document 4, if the focusing layer is formed such that the lateral sides of the glass spheres cause the incident light to be retroreflected as reflected light g2 that is substantially parallel to the incident light g1, then the problem is that incident light f1 from the front is diverged from the direction of incidence and retroreflected as reflected light f2. Therefore, even though reflective performance can be obtained at a large observation angle, such a product is far from meeting the Japanese Industrial Standards (JIS) or foreign standards for road signs, which require that reflective performance from the front be exhibited at a relatively small observation angle. Accordingly, such products have only limited applications, cannot be used for normal traffic signs, are difficult to use for vehicle license plates and so forth, and still do not meet the practical needs of the market.
As described above, conventional enclosed lens type of retroreflective sheets had high angle dependence and did not adequately ensure wide-angle reflective performance at a large incidence angle and a large observation angle.
In light of this situation, there has been an urgent need for the development of a retroreflective sheet that would meet world standards, including JIS, and provide wide-angle characteristics.